IPv6 Backbone Router
draft-ietf-6lo-backbone-router-07

Abstract

Backbone Routers placed at the wireless edge of a backbone link interconnect multiple wireless links at Layer-3 to form a large MultiLink Subnet, so that the broadcast domain of the backbone does not extend to the wireless links. Wireless nodes register or are proxy-registered to a Backbone Router to establish IPv6 Neighbor Discovery proxy services, and the Backbone Router takes care of the ND operation on behalf of registered nodes and ensures and routes towards the registered addresses over the wireless interface.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on March 7, 2019.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Applicability and Requirements Served
• 3. Terminology
• 4. Overview
• 5. Backbone Router Routing Operations
• 5.1. Over the Backbone Link
• 5.2. Over the LLN Link
• 6. Backbone Router Proxy Operations
• 6.1. Registration and Binding State Creation
• 6.2. Defending Addresses
• 7. Security Considerations
• 8. Protocol Constants
• 9. IANA Considerations
• 10. Acknowledgments
• 11. References
• 11.1. Normative References
• 11.2. Informative References
• 11.3. External Informative References
• Authors' Addresses

1. Introduction

One of the key services provided by IEEE STD. 802.1 Ethernet Bridging is an efficient and reliable broadcast service, and multiple applications and protocols have been built that heavily depend on that feature for their core operation. Unfortunately, a wide range of wireless networks do not provide economical broadcast capabilities of Ethernet Bridging; protocols designed for bridged networks that rely on broadcast often exhibit disappointing behaviours when applied unmodified to a wireless medium.

Wi-Fi Access Points (APs) deployed in an Extended Service Set (ESS) act as bridges. However, in order to ensure a solid connectivity to the devices and protect the medium against harmful broadcasts, they refrain from relying on broadcast-intensive protocols such as Transparent Bridging on the wireless side. Instead, an association process is used to register proactively the MAC addresses of the wireless device (STA) to the AP. Then, the APs proxy the bridging operation and cancel the broadcasts.

The IPv6 [RFC8200] Neighbor Discovery [RFC4861] [RFC4862] Protocol (NDP) operations are reactive and rely heavily on multicast transmissions to locate an on-link correspondent and ensure address uniqueness. When the Duplicate Address Detection [RFC4862] (DAD) mechanism was designed, it was a natural match with the efficient broadcast operation of Ethernet Bridging. However, since broadcast can be unreliable over wireless media, DAD often fails to discover duplications [I-D.yourtchenko-6man-dad-issues]. DAD usually appears to work on wireless media, not because address duplication is detected and solved as designed, but because the use of 64-bit Interface IDs makes duplication into a very rare event.

IPv6 multicast messages are usually broadcast over the wireless medium. They are processed by most if not all wireless nodes over the ESS fabric even when very few if any of the nodes are subscribed to the multicast address. Consequently a simple Neighbor Solicitation (NS) lookup message [RFC4861], that is supposedly targeted to a very small group of nodes, can consume the whole wireless bandwidth across the fabric [I-D.vyncke-6man-mcast-not-efficient]. The reactive IPv6 ND operation leads to undesirable power consumption in battery-operated devices.

The inefficiencies of using radio broadcasts to support IPv6 NDP suggest restricting broadcast transmissions over the wireless access links. This can be done by splitting the subnet in multiple ones, and in extreme cases providing a /64 per wireless device. Another way is to take over (proxy) the Layer-3 protocols that rely on broadcast operation at the boundary of the wired and wireless domains, emulating the Layer-2 association at Layer-3. Indeed, the IEEE STD. 802.11 specifications require ARP and ND proxy [RFC4389] functions at the Access Points (APs) but the specification for the ND proxy operations is still missing.

Current devices rely on snooping for detecting association state, which is unsatisfactory in a lossy and mobile conditions. With snooping, a state (e.g. a new IPv6 address) may not be discovered or a change of state (e.g. a movement) may be missed, leading to unreliable connectivity.

WPAN devices (i.e., those implementing IEEE STD. 802.15.4 [IEEEstd802154]) can make use of Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) which treats the wireless medium as different from Ethernet. RFC 6775 is updated as [I-D.ietf-6lo-rfc6775-update]; the update includes changes that are required by this document.

This specification applies to other wireless links such as Low-Power IEEE STD. 802.11 (Wi-Fi) and IEEE STD. 802.15.1 (Bluetooth) [IEEEstd802151], and extends [RFC6775] to enable proxy operation by the 6BBR. The proxy operation on the BBR eliminates the need for low-power nodes or nodes that are deep in a mesh to respond synchronously when a lookup is performed for their addresses. This provides the function of a Sleep Proxy for ND [I-D.nordmark-6man-dad-approaches].

2. Applicability and Requirements Served

Efficiency aware IPv6 Neighbor Discovery Optimizations suggests that 6LoWPAN ND [RFC6775] can be extended to other types of links beyond IEEE STD. 802.15.4 for which it was defined. The registration technique is beneficial when the Link-Layer technique used to carry IPv6 multicast packets has poor delivery ratio or requires high energy consumption in the end devices, all the more in use cases that involve mobility.

This specification updates and generalizes 6LoWPAN ND to a broader range of Low power and Lossy Networks (LLNs) with support for Duplicate Address Detection (DAD) and address lookup that does not require broadcasts over the LLNs. The term LLN is used loosely in this specification to cover multiple types of WLANs and WPANs, including Low-Power Wi-Fi, BLUETOOTH(R) Low Energy, IEEE STD. 802.11AH and IEEE STD. 802.15.4 wireless meshes, so as to address the requirements listed in Appendix B.3 of [I-D.ietf-6lo-rfc6775-update] "Requirements Related to the Variety of Low-Power Link types".

The scope of this draft is a Backbone that enable the federation of multiple LLNs into a IPv6 MultiLink Subnet. Each LLN in the subnet is anchored at an IPv6 Backbone Router (6BBR). The Backbone Routers interconnect the LLNs and advertise the addresses of the LLN nodes using proxy-ND operations. This specification extends IPv6 ND over the backbone to distinguish address movement from duplication and eliminate stale state in the backbone routers and backbone nodes once a LLN node has roamed. In this way, mobile nodes may roam rapidly from one 6BBR to the next and requirements in Appendix B.1 of [I-D.ietf-6lo-rfc6775-update]"Requirements Related to Mobility" are met.

This specification can be used by any wireless node to associate at Layer-3 with a 6BBR and register its IPv6 addresses to obtain routing services including proxy-ND operations over the backbone, providing a solution to the requirements expressed in Appendix B.4 of [I-D.ietf-6lo-rfc6775-update] "Requirements Related to Proxy Operations".

The Link Layer Address (LLA) that is returned as Target LLA (TLLA) in Neighbor Advertisements (NA) messages by the 6BBR on behalf of the Registered Node over the backbone may be that of the Registering Node. In that case, the 6BBR needs to bridge the unicast packets (Bridging proxy), or that of the 6BBR on the backbone, in which case the 6BBRs needs to route the unicast packets (Routing proxy). In the latter case, the 6BBR maintains the list of correspondents to which it has advertised its own MAC address on behalf of the LLN node. The IPv6 ND operation is minimized as the number of nodes scale up in the LLN. This meets the requirements in Appendix B.6 of [I-D.ietf-6lo-rfc6775-update] "Requirements Related to Scalability", as long has the 6BBRs are dimensioned for the number of registrations that each needs to support.

For the TimeSlotted Channel Hopping (TSCH) mode of [IEEEstd802154], the 6TiSCH architecture describes how a 6LoWPAN ND host could connect to the Internet via a RPL mesh Network, but doing so requires additions to the 6LOWPAN ND protocol to support mobility and reachability in a secure and manageable environment. This document details such additions for the 6TiSCH architecture, and serves the requirements listed in Appendix B.2 of [I-D.ietf-6lo-rfc6775-update] "Requirements Related to Routing Protocols".

In the case of Low-Power IEEE STD. 802.11, a 6BBR may be collocated with a standalone AP or a CAPWAP [RFC5415] wireless controller. Then the wireless client (STA) makes use of this specification to register its IPv6 address(es) to the 6BBR over the wireless medium. In the case of a 6TiSCH LLN mesh, the RPL root is collocated with a 6LoWPAN Border Router (6LBR), and either collocated with or connected to the 6BBR over an IPv6 Link. The 6LBR makes use of this specification to register the LLN nodes on their behalf to the 6BBR. In the case of BTLE, the 6BBR is collocated with the router that implements the BTLE central role as discussed in section 2.2 of [RFC7668].

3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Readers are expected to be familiar with all the terms and concepts that are discussed in "Neighbor Discovery for IP version 6", "IPv6 Stateless Address Autoconfiguration", "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and Goals", Neighbor Discovery Optimization for Low-power and Lossy Networks and "Multi-link Subnet Support in IPv6".

Readers would benefit from reading "Multi-Link Subnet Issues", ,"Mobility Support in IPv6", "Neighbor Discovery Proxies (ND Proxy)" and "Optimistic Duplicate Address Detection" prior to this specification for a clear understanding of the art in ND-proxying and binding.

Additionally, this document uses terminology from [RFC7102], [I-D.ietf-6lo-rfc6775-update] and [I-D.ietf-6tisch-terminology], and introduces the following terminology:

Sleeping Proxy

A 6BBR acts as a Sleeping Proxy if it answers ND Neighbor Solicitation over the backbone on behalf of the Registered Node.

Unicasting Proxy

A Unicasting Proxy forwards NS messages to the Registering Node, transforming Layer-2 multicast into unicast.

Routing proxy

A routing proxy advertises its own MAC address, as opposed to that of the node that performs the registration, as the TLLA in the proxied NAs over the backbone.

Bridging proxy

A Bridging proxy advertises the MAC address of the node that performs the registration as the TLLA in the proxied NAs over the backbone. In that case, the MAC address and the mobility of the node is still visible across the bridged backbone fabric.

Primary BBR

The BBR that will defend a Registered Address for the purpose of DAD over the backbone.

Secondary BBR

A BBR other than the Primary BBR to which an address is registered. A Secondary Router MAY advertise the address over the backbone and proxy for it.

4. Overview

An LLN node can move freely from an LLN anchored at a Backbone Router to an LLN anchored at another Backbone Router on the same backbone and keep any or all of the IPv6 addresses that it has formed.

              |
            +-----+
            |     | Gateway (default) Router
            |     |
            +-----+
               |
               |      Backbone Link
         +--------------------+------------------+
         |                    |                  |
      +-----+             +-----+             +-----+
      |     | Backbone    |     | Backbone    |     | Backbone
      |     | router      |     | router      |     | router
      +-----+             +-----+             +-----+
         o                o   o  o              o o
     o o   o  o       o o   o  o  o         o  o  o  o o
    o  o o  o o       o   o  o  o  o        o  o  o o o
    o   o  o  o          o    o  o           o  o   o
      o   o o               o  o                 o o

      LLN              LLN              LLN

    

Figure 1: Backbone Link and Backbone Routers

Each Backbone Router (6BBR) maintains a Binding Table of its Registered Nodes. The Binding Table operates as a distributed database of wireless Nodes whether they reside on the LLNs or on the backbone, and use an extension to the Neighbor Discovery Protocol to exchange that information across the Backbone as with IPv6 ND.

The Extended Address Registration Option (EARO) defined in [I-D.ietf-6lo-rfc6775-update] is used to enable the registration for routing and proxy options in the ND exchanges over the backbone between the 6BBRs to disambiguate duplication from movement.

Address duplication is detected using the ROVR field in the EARO, which is a generalization of the EUI-64 that allows different types of unique IDs beyond the name space derived from the MAC addresses. First-Come First-Serve rules apply, whether the duplication happens between LLN nodes as represented by their respective 6BBRs, or between an LLN node and a node that defends its address over the backbone with IPv6 ND and does not include the EARO.

In case of conflicting registrations to multiple 6BBRs from a same node, a sequence counter called Transaction ID (TID) in the EARO enables 6BBRs to determine the latest registration for that node. Registrations with a same TID are compatible and maintained, but, in case of different TIDs, only the freshest registration is maintained and the stale state is eliminated. The EARO also transports a 'R' flag to be used by a 6LN when registering, to indicate that this 6LN is not a router and that it will not handle its own reachability.

With this specification, Backbone Routers perform a ND proxy operation over the Backbone Link on behalf of their Registered Nodes. The registration to the proxy service is done with a NS/NA(EARO) exchange. The EARO with a 'R' flag is used in this specification to request the 6BBR to perform this proxy operation. The Backbone Router operation is essentially similar to that of a Mobile IPv6 (MIPv6) Home Agent. This enables mobility support for LLN nodes that would move outside of the network delimited by the Backbone link attach to a Home Agent from that point on. This also enables collocation of Home Agent functionality within Backbone Router functionality on the same backbone interface of a router. Further specification may extend this be allowing the 6BBR to redistribute host routes in routing protocols that would operate over the backbone, or in MIPv6 or the Locator/ID Separation Protocol (LISP) to support mobility on behalf of the nodes, etc...

The Optimistic Duplicate Address Detection (ODAD) specification details how an address can be used before a Duplicate Address Detection (DAD) is complete, and insists that an address that is TENTATIVE should not be associated to a Source Link-Layer Address Option in a Neighbor Solicitation message. This specification makes use of ODAD to create a temporary proxy state in the 6BBR till DAD is completed over the backbone. This way, the specification enables to distribute proxy states across multiple 6BBR and co-exist with IPv6 ND over the backbone.

5. Backbone Router Routing Operations

               |
            +-----+
            |     | Gateway (default) Router
            |     |
            +-----+
               | /64
               |      Backbone Link
         +-------------------+-------------------+
         | /64               | /64               | /64
      +-----+             +-----+             +-----+
      |     | Backbone    |     | Backbone    |     | Backbone
      |     | router      |     | router      |     | router
      +-----+             +-----+             +-----+
         o N * /128          o M * /128          o P * /128
     o o   o  o       o o   o  o  o         o  o  o  o o
    o  o o  o o       o   o  o  o  o        o  o  o o o
    o   o  o  o          o    o  o           o  o   o
      o   o o               o  o                 o o
  
    

Figure 2: Example Routing Configuration for 3 LLNs in the ML Subnet

5.1. Over the Backbone Link

A 6BBR is a specific kind of Border Router that performs proxy Neighbor Discovery on its backbone interface on behalf of the nodes that it has discovered on its LLN interfaces.

Some restrictions of the attached LLNs will apply to the backbone. In particular, the MTU MUST be set to the same value on the backbone and all attached LLNs. The scalability of the whole subnet requires that broadcast operations are avoided as much as possible on the backbone as well. Unless configured otherwise, in the RAs that it sends towards the LLN links, the Backbone Router MUST use the same MTU that it learns from RAs over the backbone.

On the backbone side, the 6BBR behaves like any other IPv6 router. It advertises on the backbone the prefixes of the LLNs for which it serves as a proxy.

The 6BBR uses an EARO in the NS-DAD and the multicast NA messages that it generates over the Backbone Link on behalf of a Registered Node, and it places an EARO in its unicast NA messages, if and only if the NS/NA that stimulates it had an EARO in it and the 'R' bit set.

The 6BBR SHOULD use unicast or solicited-node multicast address (SNMA) [RFC4291] to defend its Registered Addresses over the backbone. In particular, the 6BBR MUST join the SNMA group that corresponds to a Registered Address as soon as it creates an entry for that address, and as long as it maintains that entry.

Optimistic DAD (ODAD) [RFC4429] SHOULD be supported by the 6BBRs in their proxy activity over the backbone. A node supporting ODAD MUST join the SNMA of a Tentative address.

A 6BBR in Routing Proxy mode advertises the Registered IPv6 Address with the 6BBR Link Layer Address, and updates Neighbor Cache Entries (NCE) in correspondent nodes over the backbone, using gratuitous NA(Override). This method may fail if the multicast message is not properly received, and correspondent nodes may maintain an incorrect neighbor state, which they will eventually discover through Neighbor Unreachability Detection (NUD). For slow movements, the NUD procedure defined in [RFC4861] may time out too quickly, and the support of [RFC7048] is recommended in all nodes in the network.

Since the MultiLink Subnet may grow to contain many nodes, multicast should be avoided as much as possible even on the backbone. Though hosts can participate using legacy IPv6 ND, all nodes connected to the backbone SHOULD support [I-D.ietf-6man-rs-refresh], which also requires the support of [RFC7559].

5.2. Over the LLN Link

BBRs and LLN hosts on the LLN follow [RFC6775] and do not depend on multicast RAs to discover routers. LLN nodes SHOULD accept multicast RAs [RFC7772], but those are rare on the LLN link. Nodes SHOULD follow the Simple Procedures for Detecting Network Attachment in IPv6 (DNA procedures) to assert movements, and to support the Packet-Loss Resiliency for Router Solicitations to make the unicast RS more reliable.

LLN node signals that it requires IPv6 ND proxy services from a 6BBR by registering the corresponding IPv6 Address with an NS(EARO) message with the 'R' flag set. The LLN node that performs the registration (the Registering Node) may be the owner of the IPv6 Address (the Registered Node) or a 6LBR that performs the registration on its behalf.

When operating as a Routing Proxy, the BBR installs host routes (/128) to the Registered Addresses over the LLN links, via the Registering Node as identified by the Source Address and the SLLA option in the NS(EARO) messages. In that case, the MAC address of the node is not visible at Layer-2 over the backbone and the bridging fabric is not aware of the addresses of the LLN devices and their mobility. The 6BBR installs a connected host route towards the registered node over the interface to the node, and acts as a Layer-3 router for unicast packets to the node.

In that mode, the 6BBR handles the ND protocol over the backbone on behalf of the Registered Nodes, using its own MAC address in the TLLA and SLLA options in proxied NS and NA messages. For each Registered Address, multiple peer Nodes on the backbone may have resolved the address with the 6BBR MAC address and store that mapping in their Neighbor cache.

For each Registered Address, the 6BBR SHOULD maintain a list of the peers on the backbone which have associated its MAC address with the Registered Address. If that Registered Address moves to a different 6BBR, the first 6BBR SHOULD unicast a gratuitous NA(Override) to each such peer, to supply the MAC address of the new 6BBR in the TLLA option for the Address.

A Bridging Proxy can be implemented in a Layer-3 switch, or in a wireless Access Point that acts as an IPv6 Host. In the latter case, the SLLA option in the proxied NA messages is that of the Registering Node, and the 6BBR acts as a Layer-2 bridge for unicast packets to the Registered Address. The MAC address in the S/TLLA is that of the Registering Node, which is not necessarily the Registered Node. When a device moves within a LLN mesh, it may attach to a different 6LBR acting as Registering Node, and the MAC address advertised over the backbone will change.

If a registration moves from one 6BBR to the next, but the Registering Node does not change, as indicated by the S/TLLA option in the ND exchanges, there is no need to update the Neighbor Caches in the peers Nodes on the backbone. On the other hand, if the LLA changes, the 6BBR SHOULD inform all the relevant peers as described above, to update the impacted Neighbor Caches. In the same fashion, if the Registering Node changes with a new registration, the 6BBR SHOULD also update the impacted Neighbor Caches over the backbone.

6. Backbone Router Proxy Operations

This specification enables a Backbone Router to proxy Neighbor Discovery operations over the backbone on behalf of the nodes that are registered to it, allowing any node on the backbone to reach a Registered Node as if it was on-link. The backbone and the LLNs are considered different Links in a MultiLink subnet but the prefix that is used may still be advertised as on-link on the backbone to support legacy nodes; multicast ND messages are link-scoped and not forwarded across the backbone routers.

By default, a 6BBR operates as a Sleeping Proxy, as follows:

• Create a new entry in a Binding Table for a new Registered Address and ensure that the address is not a duplicate over the backbone
• Defend a Registered Address over the backbone using NA messages with the Override bit set on behalf of the sleeping node
• Advertise a Registered Address over the backbone using NA messages, asynchronously or as a response to a Neighbor Solicitation messages.
• To deliver packets arriving from the LLN, use Neighbor Solicitation messages to look up the destination over the backbone.
• Forward packets between the LLN and the backbone.
• Verify liveliness when needed for a stale registration.

A 6BBR may act as a Sleeping Proxy only for a Registered Address that is REACHABLE, or TENTATIVE in which case the answer is delayed. In any other state, the Sleeping Proxy operates as a Unicasting Proxy.

The 6BBR does not act on ND Messages over the backbone unless they are relevant to a Registered Node on the LLN side, saving wireless interference. On the LLN side, the prefixes associated to the MultiLink Subnet are presented as not on-link, so address resolution for other hosts do not occur.

As a Unicasting Proxy, the 6BBR forwards NS lookup messages to the Registering Node, transforming Layer-2 multicast into unicast. This is not possible in UNREACHABLE state, so the NS messages are multicasted, and rate-limited with an exponential back-off to protect the medium. In other states, the messages are forwarded to the Registering Node as unicast Layer-2 messages. In TENTATIVE state, the NS message is either held till DAD completes, or dropped.

The draft introduces the optional concept of primary and secondary BBRs. The primary is the backbone router that has the highest EUI-64 address of all the 6BBRs that share a registration for a same Registered Address, with the same ROVR and same Transaction ID, the EUI-64 address being considered as an unsigned 64bit integer. A given 6BBR can be primary for a given address and secondary for another address, regardless on whether or not the addresses belong to the same node. The primary Backbone Router is in charge of protecting the address for DAD over the Backbone. Any of the Primary and Secondary 6BBR may claim the address over the backbone, since they are all capable to route from the backbone to the LLN node; the address appears on the backbone as an anycast address.

The Backbone Routers maintain a distributed binding table, using IPv6 ND over the backbone to detect duplication. This specification requires that:

1. Addresses in a LLN that can be reachable from the backbone by way of a 6BBR MUST be registered to that 6BBR.
2. A Registered Node MUST include the EARO in the NS message when registering its addresses to a 6LR. The 6LR MUST forward the EARO unchanged to the 6LBR in the DAR/DAC exchange. The 6LBR MUST propagate the EARO unchanged to 6BBR.
3. The 6LR MUST respond with the same EARO in the NA, except for the status field.

A false positive duplicate detection may arise over the backbone, for instance if the Registered Address is registered to more than one LBR, or if the node has moved. Both situations are handled by the 6BBR transparently to the node. In the former case, one LBR becomes primary to defend the address over the backbone while the others become secondary and may still forward packets. In the latter case the LBR that receives the newest registration becomes primary.

Only one node may register a given Address at a particular 6BBR. However, that Registered Address may be registered to Multiple 6BBRs for higher availability.

Over the LLN, Binding Table management is as follows:

• De-registrations (newer TID, same ROVR, null Lifetime) are accepted and acknowledged with a status of 4; the entry is deleted;
• Newer registrations (newer TID, same ROVR, non-null Lifetime) are acknowledged with a status of 0 (success); the binding is updated with the new TID, the Registration Lifetime and the Registering Node; in TENTATIVE state the acknowledgement is held and may be overwritten; in other states the Registration-Lifetime timer is restarted and the entry is placed in REACHABLE state.
• Identical registrations (same TID, same ROVR) from a same Registering Node are acknowledged with a status of 0 (success). If they are not identical, an error SHOULD be logged. In TENTATIVE state, the response is held and may be overwritten, but it MUST be eventually produced and it carries the result of the DAD process;
• Older registrations (older TID, same ROVR) from a Registering Node are ignored;
• Identical and older registrations (not-newer TID, same ROVR) from a different Registering Node are acknowledged with a status of 3 (moved); this may be rate limited to protect the medium;
• Any registration for a different Registered Node (different ROVR) are acknowledged with a status of 1 (duplicate).

6.1. Registration and Binding State Creation

Upon receiving a registration for a new address with an NS(EARO) with the 'R' bit set, the 6BBR performs DAD over the backbone, placing the new address as target in the NS-DAD message. The EARO from the registration MUST be placed unchanged in the NS-DAD message, and an Neighbor Cache entry created in TENTATIVE state for a duration of TENTATIVE_DURATION. The NS-DAD message is sent multicast over the backbone to the SNMA associated with the registered address, unless that operation is known to be costly, and the 6BBR has an indication from another source (such as a Neighbor Cache entry) that the Registered Address was known on the backbone; in the latter case, an NS-DAD message may be sent as a Layer-2 unicast to the MAC Address that was associated with the Registered Address.

In TENTATIVE state after EARO with 'R' bit set:

1. The entry is removed if an NA is received over the backbone for the Registered Address with no EARO, or with an EARO with a status of 1 (duplicate) that indicates an existing registration for another LLN node. The ROVR and TID fields in the EARO received over the backbone are ignored. A status of 1 is returned in the EARO of the NA back to the Registering Node;
2. The entry is also removed if an NA with an ARO option with a status of 3 (moved), or a NS-DAD with an ARO option that indicates a newer registration for the same Registered Node, is received over the backbone for the Registered Address. A status of 3 is returned in the NA(EARO) back to the Registering Node;
3. When a registration is updated but not deleted, e.g. from a newer registration, the DAD process on the backbone continues and the running timers are not restarted;
4. Other NS (including DAD with no EARO) and NA from the backbone are not acknowledged in TENTATIVE state. To cover legacy nodes that do not support ODAD, the list of their origins MAY be stored and then, if the TENTATIVE_DURATION timer elapses, the 6BBR MAY send each such legacy node a unicast NA.
5. When the TENTATIVE_DURATION timer elapses, a status 0 (success) is returned in a NA(EARO) back to the Registering Node(s), and the entry goes to REACHABLE state for the Registration Lifetime. The 6BBR MUST send a multicast NA(EARO) to the SNMA associated to the Registered Address over the backbone with the Override bit set so as to take over the binding from other 6BBRs.

6.2. Defending Addresses

If a 6BBR has an entry in REACHABLE state for a Registered Address:

• If the 6BBR is primary, or does not support the function of primary, it MUST defend that address over the backbone upon receiving NS-DAD, either if the NS does not carry an EARO, or if an EARO is present that indicates a different Registering Node (different ROVR). The 6BBR sends a NA message with the Override bit set and the NA carries an EARO if and only if the NS-DAD did so. When present, the EARO in the NA(Override) that is sent in response to the NS-DAD(EARO) carries a status of 1 (duplicate), and the ROVR and TID fields in the EARO are obfuscated with null or random values to avoid network scanning and impersonation attacks.
• If the 6BBR receives an NS-DAD(EARO) for a newer registration, the 6BBR updates the entry and the routing state to forward packets to the new 6BBR, but keeps the entry REACHABLE. Afterwards, the 6BBR MAY use REDIRECT messages to reroute traffic for the Registered Address to the new 6BBR.
• If the 6BBR receives an NA(EARO) for a newer registration, the 6BBR removes its entry and sends a NA(EARO) with a status of 3 (MOVED) to the Registering Node, if the Registering Node is different from the Registered Node. The 6BBR cleans up existing Neighbor Cache Entries in peer nodes as discussed in Section 5.1, by unicasting to each such peer, or one broadcast NA(Override).
• If the 6BBR receives a NS(LOOKUP) for a Registered Address, it answers immediately with an NA on behalf of the Registered Node, without polling it. There is no need of an EARO in that exchange.
• When the Registration-Lifetime timer elapses, the entry goes to STALE state for a duration of STABLE_STALE_DURATION in LLNs that keep stable addresses such as LWPANs, and UNSTABLE_STALE_DURATION in LLNs where addresses are renewed rapidly, e.g. for privacy reasons.

The STALE state enables tracking of the backbone peers that have a Neighbor Cache entry pointing to this 6BBR in case the Registered Address shows up later. If the Registered Address is claimed by another node on the backbone, with an NS-DAD or an NA, the 6BBR does not defend the address. In STALE state:

• If STALE_DURATION elapses, the 6BBR removes the entry.
• Upon receiving an NA(Override) the 6BBR removes its entry and sends a NA(EARO) with a status of 4 (removed) to the Registering Node.
• If the 6BBR receives a NS(LOOKUP) for a Registered Address, the 6BBR MUST send an NS(NUD) following rules in [RFC7048] to the Registering Node targeting the Registered Address prior to answering. If the NUD succeeds, the operation in REACHABLE state applies. If the NUD fails, the 6BBR refrains from answering the lookup. The NUD SHOULD be used by the Registering Node to indicate liveness of the Registered Node, if they are different nodes.

7. Security Considerations

This specification applies to LLNS in which the link layer is protected, either by means of physical or IP security for the Backbone Link or MAC sublayer cryptography. In particular, the LLN MAC is required to provide secure unicast to/from the Backbone Router and secure Broadcast from the Backbone Router in a way that prevents tempering with or replaying the RA messages.

The use of EUI-64 for forming the Interface ID in the link local address prevents the usage of Secure ND ([RFC3971] and [RFC3972]) and address privacy techniques. This specification RECOMMENDS the use of additional protection against address theft such as provided by [I-D.ietf-6lo-ap-nd], which guarantees the ownership of the ROVR.

When the ownership of the ROVR cannot be assessed, this specification limits the cases where the ROVR and the TID are multicasted, and obfuscates them in responses to attempts to take over an address.

8. Protocol Constants

This Specification uses the following constants:

TENTATIVE_DURATION:

800 milliseconds

STABLE_STALE_DURATION:

24 hours

UNSTABLE_STALE_DURATION:

5 minutes

DEFAULT_NS_POLLING:

3 times

9. IANA Considerations

This document has no request to IANA.

10. Acknowledgments

Kudos to Eric Levy-Abegnoli who designed the First Hop Security infrastructure at Cisco.

11. References

11.1. Normative References

11.2. Informative References

11.3. External Informative References

Authors' Addresses